Even the smallest vacuum leak on an internal combustion engine can prevent it operating to the level of its capability.
No matter how well you rebuild the carburetor, adjust the timing, or clean the gas tank, the engine will continue to operate below what it should even with a small vacuum leak.
On older engines a quick, easy way to check for leaks near the cast iron manifold will often uncover the problem.
- Place a large piece of cardboard behind the radiator cooling fan. See photo. This stops fan air flow in the area of the meeting point of the manifold, carburetor, and engine head.
- Use a spray can of starting fluid (available at auto parts stores). Let engine idle, and lightly spray in areas of where air flow passes through areas below where air flows from the carburetor to where the intake manifold attaches to the engine head.
- If you have even a small vacuum leak the starting fluid will be pulled incorrectly into the engine combustion chamber. The engine RPM will instantly increase. Your engine problem has been found! A new gasket or insulator plate will usually make all well.
